In Silverlight and WPF, there are several ways you can create animations. One way that has been covered before on this site involves creating a storyboard and using Expression Blend to visually create your animation. This approach is great for majority of all animations you would ever want to create. There are exceptions where this solution doesn't work as well as you would like.
Animations that are more random or will have a certain degree of variability don't fit well inside the box defined by Storyboards. For example, here is an example of an animation where the circles move around a centerpoint with a random speed each time you refresh the page:
[ the circles...they are spinning! ]
Something similar to what you see above works best when created in code, and this tutorial will show you one way of going about doing that. http://www.kirupa.com/blend_silverlight/animations_code_pg1.htm